Thyroid cancer is the most common endocrine malignancy and its incidence is the second fasting rising of all cancers in the United States. Therapy for localized thyroid cancer is effective, but patients with distant metastasis or locally aggressive tumors have a poor prognosis. Despite major advances in defining the pathogenic mechanisms for thyroid cancer development, the pathways responsible for thyroid cancer progression remain poorly defined. Thus, characterizing pathways that inhibit thyroid cancer invasion and metastasis represents an important goal for developing targeted therapies for this disease. GPR54 has been characterized to be the receptor for an endogenously produced metastasis inhibiting peptide, metastin (KiSS-1 gene product). In overexpression models, GPR54 is a G-alpha q-11-coupled receptor that activates phospholipase C and other pathways, resulting in metastasis inhibition. More recently, patients with inactivating mutations in the GPR54 gene and GPR54 null mice have been shown to develop hypogonadotropic hypogonadism through uncertain mechanisms. In thyroid cancer, we have demonstrated that GPR54 is overexpressed in papillary thyroid cancer, but is lost in follicular thyroid cancer, the subtype with the greatest predilection for distant metastasis. We have shown that, in addition to PLC activation, the endogenously expressed GPR54 aIso activates E RKI/2, but not p38 MAPK in thyroid cancer cells. Because PLC, ERK and Akt activation induce cell proliferation and migration, we reasoned that GPR54 must also interact with other pathways to exert its inhibitory effect. To identify GPR54-altered genes that might be related to these properties, we performed cDNA array studies on total RNA isolated from GPR54-expressing thyroid cancer cells after 1 and 24 hours of metastin stimulation. In all experiments (duplicate RNA batches for each time point), and at both time points, the gene encoding MCIP1 (calcineurin inhibitor) was upregulated. These data have been confirmed by quantitative RT-PCR and functional studies have demonstrated that metastin down-regulates calcineurin activity in thyroid cancer cells. The purpose of this grant application is to determine if inhibition of calcineurin and upregulation of MCIP1 are involved in the anti-proliferative and migratory effects of metastin, and if they represent targets for therapy of this aggressive and frequently fatal disease.